As one of methods for charging the surface of an electrophotographic photosensitive member, a contact charging method is currently practical.
The contact charging method is a method in which a voltage is applied to a charging member situated to be in contact with the electrophotographic photosensitive member to cause a very low level of electrical discharge near a contact area between the charging member and the electrophotographic photosensitive member, whereby the surface of the electrophotographic photosensitive member is charged.
For the charging member for charging the surface of the electrophotographic photosensitive member, those comprising a support and an elastic layer (conductive elastic layer) provided on the support are commonly used in terms of securing a nip of contact between the electrophotographic photosensitive member and the charging member.
The elastic layer (conductive elastic layer) often contains a relatively large amount of low molecular weight components, and therefore for the purpose of inhibiting the low molecular weight components from bleeding out to contaminate the surface of the electrophotographic photosensitive member, the conductive elastic layer is often provided thereon with a surface layer which is different from the conductive elastic layer and has an elastic coefficient smaller than that of the conductive elastic layer.
As for the shape of the charging member, roller-shaped charging members are commonly used. Hereinafter, the roller-shaped charging member is also referred to as “charge roller”.
The method which is most widely used among contact charging methods is a method in which a voltage with an alternating current voltage superimposed on a direct current voltage is applied to the charging member (hereinafter also referred to as “AC+DC contact charging method”). In the case of the AC+DC contact charging method, a voltage having a peak-to-peak voltage twice or more than twice as high as a charge starting voltage is used for the alternating current voltage.
The AC+DC contact charging method is a method enabling highly uniform and stable charge to be done by using the alternating current voltage, but use of an alternating current voltage source incurs upsizing of a charging apparatus and an electrophotographic apparatus and an increase in cost compared with a method in which a voltage with only a direct current voltage is applied to the charging member (hereinafter also referred to as “DC contact charging method”).
Namely, the DC contact charging method is a charge method which is excellent in terms of downsizing of the charging apparatus and the electrophotographic apparatus and a reduction in cost compared with the AC+DC contact charging method.
Japanese Patent Application Laid-Open No. 2003-107927 discloses a transfer member having a dynamic friction coefficient of 0.4 or less and surface free energy of 35 dyn/cm or less.